1. Field of the Invention
The present invention relates to a gearing apparatus for transmitting a rotational motion between two shafts by assembling a pair of resin-made toothed wheels (it will be referred to as gears hereinafter) to engage opposite teeth with each other, and in particular, to a resin-made gearing apparatus for absorbing a given amount of backlash appearing upon engagement of the pair of gears by a backlash absorbing section and also for reducing the deformation and stress of the backlash absorbing section.
2. Description of the Related Art
The resin-made gearing apparatus of this type is characterized in that it is lightweight compared with a metal gearing apparatus, and also is capable to be used without the lubrication due to the self-lubricity thereof and in low noise. Further, the resin-made gearing apparatus, such as a molded plastic gear or the like, has a characteristic of high mass-production to achieve the reduction of product cost. Therefore, in recent years, the molded plastic gear has been widely used for a gear train in an ink jet printer, a gear train in a power transmission section of an automotive component or in a precision machine or instrument, and the like.
Here, generally, between teeth oppositely mating with each other in the assembly of a pair of gears, a clearance (play) called a backlash is disposed. This backlash is disposed for preventing the friction of a tooth surface of the gear due to a tooth profile error, an assembly error or the like, and also, for smoothing the meshing of the gear teeth.
However, in the recent gearing apparatus, the significantly high accuracy in the transmission of rotational motion is often required. For example, in certain types of angle sensors, it is required to transmit to the output side a detected angle at an absolute accuracy without involving any error. In this case, a backlash in the pair of meshed gears causes generation of an error in the transmission of rotational motion. Accordingly, there is a demand for a gearing apparatus whose backlash is constantly nil irrespective of an operative temperature environment or a dimensional error of the gears. As one measure for satisfying such a demand, a gear, which is free of backlash (it will be referred to as a backlash-less gear hereinafter) has conventionally been proposed.
According to the conventional backlash-less gear, as shown in FIG. 10 for example, a gear 50 is formed with a fin portion 52 on each of both tooth flanks on one end side in a tooth width direction of each tooth 51, to be in contact with a mated or meshed gear, thereby absorbing any shock at the time coming into mating contact by the deformation of the fin portion 52 (refer to Japanese Unexamined Utility Model Publication No. 55-100745 (FIG. 1 and FIG. 3)).
However, in the above described conventional backlash-less gear, the elastically deformable fin portion 52 is formed only on each end portion in the tooth width direction of each tooth, in one of the assembly of the pair of gears, but any contrivance is not provided on each tooth of the other meshed gear. Therefore, the fin portion 52 on the tooth in the one gear may be in direct contact with a tooth flank of the tooth of the other meshed gear so that both gears must be subjected to a high stress. Namely, although the fin portion 52 on each of the end portions in the tooth width direction in the one gear is elastically deformable, the tooth flank of each tooth in the other meshed gear forms a power transmitting plane which is not elastically deformed. Hence, the fin portion 52 in the one gear may come in contact with the power transmission plane, to be deformed to an extent exceeding an elastic limit range. In particular, in the case where the fin portion 52 in the one gear is consecutively formed along the periphery of the tooth profile to be folded in a U-shape at a tooth crest portion, the high stress mainly acts on the folded portion, and in an extreme case, it is deemed that the fin portion 52 of the tooth in the one gear is broken.